The bomb blast in London yesterday is almost certain to force the police and security agencies to bring back much of the capital's protective "ring of steel".
Security measures for key targets, such as senior politicans, police stations, and Downing Street, will have been stepped up immediately.
There will also be a tightening of security in public areas, and potential economic targets - such as Europe's tallest tower block, Canary Wharf - and mainline railway stations.
In the 17 months since the ceasefire there has been a gradual running down of anti-terrorist precautions. Ministers and other public figures are no longer so closely guarded; public buildings, such as stations, have relaxed their security.
But yesterday's blast, if it is found to be the work of the IRA, will reverse all those changes.This will have a knock- on effect on the UK economy and the lifestyle of millions of citizens.
The blast follows recent warnings from intelligence and security officers that the IRA has assembled a "hit list", which would target leading politicians for assassination.
Although yesterday's explosions suggests that economic targets and "big bombs'' - such as those which hit the City of London in 1992 and 1993 could also be on the agenda.
MI5 and anti-terrorist officers have recently warned the Government that the IRA would strike quickly at targets on the mainland if the ceasefire broke down.
Protection on some ministerial cars, which have bullet proof windows, has been upgraded to include sophisticated electronic sensing devices which alert the driver if a bomb has been attached to the underside.
The warnings were publicly aired by Sir Hugh Annesley, the Chief Constable of the Royal Ulster Constabulary, who said last weekend that if there was a return to violence, the campaign would be directed at mainland Britain.
The immediate effect of a resumption of terrorism in the UK is likely to be the kind of police activity that was commonplace until the ceasefire: random stop and search, armed roadblocks and police patrols.
In the 17 months since the ceasefire, many anti-terrorist and Special Branch officers in London and Belfast - whose numbers were at an all time high - have been moved into normal policing work or transferred to other duties - such as animal rights, Middle Eastern terrorism and to anti-drugs work.
MI5 which has successfully taken a lead role in terrorism intelligence- gathering over the police, only to then see the need downgraded, will now have to realign itself.Reuse content